One way of analyzing a biological sample is to use photometry. Typically the spectra of scattered light or the transmission at certain wavelengths is measured. Not all molecules have a scattering or transmission spectra which is useful. Solutions containing a reagent may be mixed into the biological sample to alter its spectral properties. Different reagents may be selected which exhibit changes in spectral properties depending upon a physical property such as the concentration of a particular molecule or analyte. In order to make these measurements useful the changes in the spectral properties as a function of the physical property should be calibrated.